The term “mass” here refers to the “charge-related mass” m/z, which is the only quantity that can be measured in mass spectrometry, and not simply the “physical mass” m. The number z is the number of elementary charges, i.e., the number of excess electrons or protons of the ion, which act externally as the ion charge. The charge-related mass is the mass fraction of the ion per excess elementary charge.
The term “ions” here refers to all charged particles; in this sense, electrons are also ions, for example, with a tiny mass of only m=1/1823 daltons.
For around three decades, RF multipole rod systems have been used both as ion storage devices and as ion guides. Particularly well known are RF quadrupole rod systems according to Wolfgang Paul with four pole rods, but hexapole and octopole rod systems are also frequently used, depending on the requirements regarding the radial bundling of the ions. The rod systems can consist of round pole rods, but for the generation of ideal fields, rods with hyperbolic shapes must be used.
The effect of the multipole systems is described by so-called “pseudopotentials”, fictitious potentials which make it possible to describe the effect of inhomogeneous alternating fields on ions in a simple way. An alternating field at the tip of a wire, whose strength decreases at 1/r2, or an alternating field around a long wire, which decreases at 1/r, reflects both positively and negatively charged particles. This occurs because the particle oscillates in the alternating field of the wire. Irrespective of its charge, the particle experiences maximum repulsion from the wire precisely when it is at the point of its oscillation that is closest to the wire, i.e., at the point where the field strength is highest. The particle experiences maximum attraction when it is furthest away, i.e., at the point on its trajectory where the field strength is lowest. Integration over time therefore gives a repulsion of the particle, which is permanently oscillating in the RF field, away from the tip. The repulsive field obtained by integration over time is described by this fictitious “pseudopotential”, which is proportional to the square of the alternating field strength. The derivative of this gives an electric “pseudo force field”. For the tip of the wire, the repulsive pseudopotential decreases at 1/r4; for the long wire it decreases outward at 1/r2, but in both cases it is still inversely proportional to the mass of the ions and likewise inversely proportional to the square of the frequency. Ions of different charge-related mass m/z thus experience repulsions of different strengths; heavier ions are repelled less strongly.
If one examines the pseudopotential in the cross-section of a quadrupole rod system, it approaches zero in the axis of the rod system. The pseudopotential increases quadratically from the axis outward in all radial directions. The rotationally symmetric parabolic minimum of the pseudopotential in the cross-section forms a potential well along the axis of the rod system. Ions of low kinetic energy can oscillate harmonically in the radial direction through the potential well or they can orbit or tumble around the potential well. If a rod system such as this is filled with a collision gas at a pressure between 0.01 and 1 pascal, ions injected with a few electronvolts give up most of their kinetic energy as a result of collisions with this gas in a short period of time of only 0.1 to 10 milliseconds and collect as a thin string of ions only with thermal energy in this potential well along the axis. The collision gas is therefore also referred to as damping gas. The diameter of the ion string depends on the mutual repulsion of the ions, which opposes the centripetal force of the pseudopotential. This focusing effect can also be observed when the ions are transported through a gas-filled multipole system. This process, described already in German Patent DE 27 01 395, is now called “collision focusing”.
Collision focusing is of major importance for most modern mass spectrometers. The injection of ions into a subsequent stage of a mass spectrometer, for example into a subsequent vacuum stage, ion guide or ion analyzer, almost always depends on the cross-section of the ion beam. A very fine beam cross-section, as is produced by collision focusing, is almost always advantageous. This applies for injection into a quadrupole mass filter just as it does for injection into an ion trap, and most particularly for injection into a time-of-flight mass spectrometer (OTOF), which pulses out ions of a fine ion beam by a pulser, orthogonally to the previous flight direction, into the flight path; here the good shaping of a fine primary beam is essential for the resolving power of the OTOF.
The rod systems used to guide ions are generally very long and thin so that they can concentrate the ions in a region with a very small diameter. They can then advantageously be operated with low RF voltages and form a good starting point for the subsequent ion-optical imaging of the ions. The cylindrical interior often has a diameter of only around 2 to 4 millimeters, the rods are less than a millimeter thick, and the system is 2 to 25 centimeters long. They are mainly used to guide ions through the various chambers of differential pump systems. The term “long” pole rods here should be taken to mean pole rods which are longer than the separation between opposite pole rods.
The rod systems used as collision cells for collision-induced fragmentation are usually not as slim; they usually have internal rod separations of 6 to 8, sometimes up to 12, millimeters in order to keep the ions, which diffuse laterally due to the statistically acting collisional deflection, in the collision cell. Similar considerations apply to reaction cells, in which positive and negative ions are made to react. These also require special terminations at the ends in order to keep ions of both polarities within the reaction cell.
It is known that all RF rod systems show a lower mass limit for the storage or transmission of ions. In quadrupole rod systems this mass limit is sharply defined, but less so in higher multipole systems. The mass limit depends on the frequency and amplitude of the RF voltage. It is inversely proportional to the square of the frequency and linearly proportional to the amplitude. For a predetermined frequency, it is therefore the amplitude of the RF voltage which determines the lower mass limit. If light ions are also to be transmitted without losses, the amplitude of the RF voltage must be chosen so as to be small. The lower mass limit is given by the stability zone of the Mathieu differential equation for the motion of the ions in RF quadrupole fields. A pseudopotential cannot form for light ions because a pseudopotential requires an integration over several periods of the RF voltage, but these light ions are accelerated in just a half-period of the RF voltage to such a degree that they are either propelled out of the storage field in a single half-period, or they experience this propulsion by being excited increasingly in a few half-periods.
Electrons cannot be stored in conventional systems because their mass, which is only around 1/2000 of the mass of a proton, is far below the lower mass limits which can usually be set. The lower mass limit is usually set to between 50 and 300 daltons, and in rare cases lower.
The fact that quadrupole rod systems have an upper mass limit is less well known. The Mathieu differential equations state only that the restoring forces of the pseudopotential are smaller for heavy ions than for light ions. The restoring forces are proportional to the inverse z/m of the charge-related mass m/z of the ion. This means that light ions collect in the axis because the focusing pseudopotential is stronger for them, with higher filling rates heavier ions are forced to gather outside the axis, kept at a distance from the lighter ions by Coulomb repulsion.
When a quadrupole rod system is used as an ion storage device, the upper mass limit only makes itself felt during the injection and if the rod system is overfilled. Even if the injection is only slightly oblique, the weak pseudopotential for heavy ions can no longer deflect them back to the axis; they hit the pole rods or overcome the potential saddles of the spaces between the pole rods and are eliminated. If the system is overfilled, the space charge drives the heavy ions right up to the pole rods or over the potential saddles. If the quadrupole rod system is filled with a collision gas, there are two further components to consider: the thermal diffusion brought about by gas collisions, which can drive heavy ions out of the rod system because of the weak pseudopotential opposing field, and the collision cascades experienced by ions injected at high energy, whose lateral angles of deflection can randomly add up, with the result that the ions impact on the pole rods or can escape through the gap between the pole rods. Both effects result in considerable losses of heavy ions. Furthermore, heavy ions are discriminated if ions are axially ejected from the ion guide, because they are not in the axis.
The upper mass limit is not sharply defined, but it does attenuate the intensity of heavy ions to such a degree that they can no longer be readily detected by a mass spectrometer. The rule of thumb for a quadrupole rod system is that when an ion mixture is injected, the ions whose masses m/z are greater than twenty times the lower mass limit are attenuated by losses to such a degree that they can no longer be readily measured, especially no longer true to concentration. These heavy ions can even disappear completely, depending on the mixture of the ions in the quadrupole rod system.
The existence of the upper mass limit is already inconvenient in the field of peptide analysis in proteomics. The aim here is to measure not only the ions of individual amino acids, the so-called “immonium ions”, but also the mass range of the so-called digest peptides up to around 5000 daltons. But if the lower mass limit for the measurement of the immonium ions is set to around 50 daltons, the rule of thumb states that this results in an upper mass limit of around 1000 daltons, which is completely unacceptable for this type of analysis. This means that time-of-flight mass spectrometers with orthogonal ion injection, which are employed particularly because of their high mass range, cannot be adequately used.
One solution is to use hexapole or octopole rod systems. These have more favorable pseudopotential distributions for heavier ions, with a steeper potential increase outside the axis in front of the pole rods, but with a flatter base of the potential well close to the axis. The pronounced pseudopotential minimum which exists in the axis of a quadrupole field does not exist here. However, this means that the ions do not collect as accurately in the axis of these systems and can thus no longer be injected as favorably into subsequent systems. The collision focusing is weaker. The operation of time-of-flight mass spectrometers with orthogonal ion injection suffers from a poorer resolution because the required fine cross-section of the ion beam can no longer be achieved.
Particularly in octopole rod systems, if the system is filled with a large quantity of ions, the heavier ions may collect far outside the axis, very close to the rods, because they are driven thereto by the space charge. This charge-dependent distribution of the ions in the interior is very unfavorable. It can even occur when there are no light ions at all in the ion mixture; the pure Coulomb repulsion between the heavy ions is sufficient. The ions collect on the surface of a cylinder; no collision focusing takes place at all if a limit ion density is exceeded.
Similarly, the limited mass range is unfavorable for those multipole systems in which reactions between very light negative reactant ions and heavy, multiply charged positive ions are to take place. In order to introduce the light reactant ions, the RF amplitude must be decreased to such an extent that losses of heavy ions occur. According to the current prior art, it is quite unfeasible to store heavy ions and electrons simultaneously.
There are publications concerned with the expansion of the mass range, in particular for ion guides. In these cases, attempts are made to force a stronger repulsion for heavy ions in the outer region near the pole rods. International Application WO 2001/013100 A2 discloses RF voltages with at least two frequencies are applied to a multipole rod system so that an RF field with lower frequency is generated in the immediate vicinity of the pole rods in order to drive the heavy ions back. Superimposed on this RF field is a quadrupolar RF field of higher frequency which collects light ions in the center. U.S. Pat. No. 7,595,486 describes how the usable mass range for the ions can be increased by giving the electrodes of rod systems a finer mechanical structure and by an appropriate electrical configuration.
A simultaneous storage of ions from extremely different mass ranges, for example electrons and heavy ions, is not remotely achievable with these measures.
There is a need for an arrangement with which, at least in radial direction, charged particles from extremely different mass ranges, for example electrons and heavy positive ions, can be retained in order to react with each other.